1. Field of the Invention
The present invention relates to a drive circuit for driving a current-driven display unit using organic electroluminescent devices (hereinafter called xe2x80x9cEL devicesxe2x80x9d), light-emitting diodes (hereinafter called xe2x80x9cLEDsxe2x80x9d), etc. which respectively emit light according to the supply of currents.
This application is counterpart of Japanese patent applications, Ser. No. 328997/2001, filed Oct. 26, 2001, the subject matter of which is incorporated herein by reference.
2. Description of the Related Art
FIG. 1 is a circuit diagram showing the outline of a general display unit using EL devices.
The present display unit principally comprises a display panel 1001, a scan line drive circuit 1003, a data line drive circuit 1005, and a control circuit 1007.
The display panel 1001 has a plurality of scan lines COM1 through COMn, a plurality of data lines SEG1 through SEGm, and a plurality of EL devices EL11 through ELnm respectively placed at points where the scan lines and the data lines intersect one another.
The scan line drive circuit 1003 comprises a plurality of switch means SWc1 through SWcn respectively electrically connected to the plurality of scan lines COM1 through COMn. The switch means SWc1 through SWcn electrically connect their corresponding scan lines COM1 through COMn to either a ground potential GND (e.g., 0V) or a scan line source potential Vc (e.g., 20V).
The data line drive circuit 1005 principally comprises a plurality of switch means SWs1 through SWsm respectively electrically connected to the plural data lines SEG1 through SEGm, and a plurality of constant current devices CC1 through CCm. The switch means SWs1 through SWsm electrically connect their corresponding data lines SEG1 through SEGm to the ground potential GND or the constant current devices CC1 through CCm. The constant current devices CC1 through CCm are respectively electrically connected to the data line source potential Vs (e.g., 20V).
The control circuit 1007 controls the operations of the switch means SWc1 through SWcn and the switch means SWs1 through SWsm, based on control data.
The states of the individual switch means at the time that only the EL device EL11 is in a light emitting state, are shown in FIG. 1. The light emitting state and non-light emitting state of the EL device will be described below in brief.
The cathode of the EL device EL11, i.e., the scan line COM1 is supplied with the ground potential GND by the switch means SWc1 of the scan line drive circuit 1003. Incidentally, when the ground potential GND is supplied to the corresponding scan line, the scan line is defined as an active state or a selected state. On the other hand, when the scan line source potential Vc is supplied thereto, it is defined as an inactive state or a non-selected state. Accordingly, the scan line COM1 is in an active state at present.
On the other hand, the anode of the EL device EL11, i.e., the data line SEG1 is supplied with the data line source potential Vs by the switch means SWs1 of the data line drive circuit 1005. Since the EL device EL11 is biased in the forward direction in this condition, a current path extending from the data line source potential Vs to the ground potential GND is formed. Thus, such a current I1 as shown in the drawing flows through the EL device EL11. Owing to the flow of the current I1 through the EL device EL11 in this way, the EL device EL11 is allowed to transition to the light emitting state.
The cathode of the EL device EL21, i.e., the scan line COM2 is supplied with the scan line source potential Vc by the switch means SWc2 of the scan line drive circuit 1003. Since there is no difference in potential between the anode and cathode of the EL device EL21 in this condition, a current path extending from the data line source potential Vs to the ground potential GND is not formed. Thus, since no current I1 flows through the EL device EL21, the EL device EL21 does not change to the light emitting state.
The cathode of the EL device EL12, i.e., the data line SEG2 is supplied with the ground potential GND by the switch means SWs2 of the data line drive circuit 1005. Since the anode of the EL device EL12 is not supplied with a current through the constant current device CC2, current I1 does not flow through the EL device EL12 and hence the EL device EL12 is not caused to transition to the light emitting state.
Similarly, the cathode of the EL device EL22, i.e., the data line SEG2 is supplied with the ground potential GND by the switch means SWs2 of the data line drive circuit 1005. Further, the cathode of the EL device EL22, i.e., the scan line COM2 is supplied with the scan line source potential Vc by the switch means SWc2 of the scan line drive circuit 1003. Since the EL device EL22 is biased in the reverse direction in this condition, no current I1 flows through the EL device EL22 and hence the EL device EL22 is not transitioned to the light emitting state.
While each of the EL devices is caused to transition to the light emitting state by being supplied with the current as described above, the amount of light emitted therefrom (the degree of light emitted) depends on a current value. When the amount of light emitted from each EL device falls outside a predetermined set value (corresponding to a standardized value set in consideration of an error), an intended display cannot be realized. Thus, the current values supplied to the respective data lines need to be constant values equal to one another. In order to keep constant the current values supplied to the data lines, the data line drive circuit 1005 is provided with the constant current devices CC1 through CCm. The constant current devices CC1 through CCm are supplied with a constant voltage at their gates, for example, and comprise MOS transistors operated in their saturated regions.
Since, however, variations and errors in manufacturing exist, the characteristics of all the MOS transistors that function as the constant current devices, do not necessarily fall within the set values (standardized values set in consideration of the errors). While, for example, a threshold voltage exists as one parameter indicative of the characteristic of each MOS transistor, a current Ids flowing between the drain and source of the MOS transistor also falls outside a set value where the threshold voltage takes values different from one another every MOS transistors respectively constituting the constant current devices. Thus, the current values supplied to the respective data lines are not brought to constant values equal to one another and vary each other. As a result, a problem arises in that the amounts of light emitted from the EL devices will vary every data lines.
Accordingly, there has been a demand for the advent of an improved drive circuit hard to be affected by the variations in manufacturing.
According to one aspect of the present invention, there is provided a drive circuit that includes an input node for receiving data, an output node. The drive circuit also includes a first MOS transistor of a first conductivity type and a second MOS transistor of the first conductivity type. The first MOS transistor has a source, a drain connected to the output node, and a gate connected to the input node. The second MOS transistor has a source, a drain connected to the source of the first MOS transistor, and a gate supplied with a predetermined potential level. The drive circuit also includes resistance means connected between the source of the second MOS transistor and a source node supplied with a source potential level.